The use of wooden structural supports as building components is common, particularly in the western United States. Common wooden structural supports include rafters, joists, beams, and headers (beams built-up from multiple layers of lumber). There are many factors to be considered in designing a structure using wooden supports. Among the factors are the size of the structural supports, i.e., thickness and width, and the span across which they extend. The required size depends upon the loads placed upon the structural supports, the type and grade of wood used, and the span of the structural supports. Conversely, the permissible span will depend upon the loads, the type and grade of the wood, and the size of tie structural supports. Building codes, which may specify, for example, a minimum factor of safety or a maximum designed deflection, are also a factor in determining the design and type of wood used in structural supports.
A designer can predetermine and fix several of these factors and then calculate the remaining design parameters. For example, a designer can decide tie loads that will be supported, the type and grade of the wood used, the size of the supports, and the maximum deflection permitted, then determine the maximum span possible under those conditions. Alternatively, the designer can specify a predetermined load, grade and type of wood, maximum permitted deflection, and span, then calculate the size of supports required.
Calculation of any of the above factors typically requires the use of tabulated data for the different grades and species of wood. Tabulated data of this sort is based on empirical test results obtained in accordance with ASTM standards D2555 and D245 for clear wood and ASTM D1990 for full size test specimens. For western U.S. species of wood tables are published by Western Wood Products Association, Portland, Oreg., in the WWPA Western Lumber Product Use Manual and by the American Forest and Paper Association, Washington, D.C., in the AF&PA National Design Specification Supplement. Because of the various combinations of wood types, grades, and use constraints, a large number of tables are required. Because of the complexity of the calculations and the tables, design calculations are difficult to perform in the field, for example, at a construction site.
Sophisticated computer programs for use with a general purpose personal computer or minicomputer exist for the designer of wooden structures. Such programs are typically rather complex and require the user to have a detailed knowledge of the engineering of structural designs. Furthermore, the physical size of the personal computers required to operate these computer programs is prohibitive of the use of these programs at a construction site.